1. Field of the Invention
The present invention relates to a process for producing acrylic acid by vapor phase catalytic oxidation of propylene in two steps. More particularly, the invention relates to a process for producing an aqueous solution of acrylic acid of a high concentration by oxidation of propylene at a high concentration and also providing with prolonged catalytic activity a Mo-Bi composite oxide catalyst used for vapor phase catalytic oxidation in the first step.
2. Discussion of the Background
The process for producing acrylic acid by vapor phase catalytic oxidation in two steps using molecular oxygen is already known and used on an industrial scale.
One of the typical processes for industrial production is as follows. Propylene is converted mainly into acrolein and a small amount of acrylic acid in the first step by supplying a mixture of propylene, air and steam, and the product is supplied to the second reactor without separation of products. In this case, the method of adding a new air and steam required for the subsequent reaction in the second step, at the inlet of the second reactor is also known and described, for example, in Japanese Patent Laid-Open Publication No. 73041/1981.
Another typical process is that wherein the product gas containing acrylic acid obtained at the outlet of the second reactor is introduced into a collecting apparatus to obtain acrylic acid as an aqueous solution and a part of remaining waste gas containing unreacted propylene coming out from the collecting apparatus is reused by adding it at the first reactor inlet together with the starting gas mixture of propylene, air and steam. A part of unreacted propylene is recycled in this method, which is described in, for example, Japanese Patent Publication No. 30688/1978 and Japanese Patent Laid-Open Publication Nos. 108917/1977, 15314/1978 and 36415/1976.
These two typical processes are classified into two major categories, the former being a process without recycling (hereinafter referred to as the non-recycling process or one-pass process) and the latter being one recycling the waste gas containing unreacted propylene (hereinafter referred to as the unreacted propylene recycling process).
A lot of improved methods of the non-recycling and unreacted propylene recycling processes have been proposed respectively in the references cited above to produce in a large scale acrylic acid efficiently by vapor phase catalytic oxidation of propylene. Among all these proposals, the most important issue is how to increase the space time yield of acrylic acid as gas and the acrylic acid concentration in the condensate obtained.
On the other hand, it is apparent that the economy of these processes is greatly influenced by the performance of the oxidation catalysts used, and many proposals with respect to catalysts have been presented, for example, in Japanese Patent Publication Nos. 17711/1972, 27490/1972, 41329/1972, 42241/1972, 42813/1972, 1645/1973, 4763/1973, 4764/1973, and 4765/1973 for the first step catalysts, and Japanese Patent Publication Nos. 12129/1969, 19296/1973, 169/1974, 11371/1974, 10432/1977, 31326/1977 and Japanese Patent Laid-Open Publication Nos. 2011/1971, 8360/1972 and 43922/1974 for the second step catalysts.
The currently used catalysts for industrial production are mainly Mo-Bi composite oxide catalysts for the first step and Mo-V composite oxide catalysts for the second step.
There are many reasons why the characteristics of these oxidation catalysts greatly affect the economy of the processes. Primarily the selectivity of the catalysts for the reactions affects the quantity of propylene used, and the catalyst activity in the reactions affects the space time yield of acrylic acid. Furthermore, the catalyst life and its price affect the total catalyst cost. Further, the applicability of the catalysts to various reaction conditions affects the quantity of energies used for the reactions.